In the case in which human effluent, such as urine, and living wastewater, each of which is generated in a closed system space, such as a nuclear shelter, a hazardous shelter, a space station or a moon-Mars mission manned spacecraft, or a lunar base, are treated in this closed system space for water recovery, there are following restrictions.
1) Since the gravity is small in a cosmic space or the like, gas-liquid separation and solid-liquid separation using the gravity are difficult to be performed.
2) Since the space is a closed system space, emission gas species and the emission amount thereof are limited.
3) A high water recovery rate is required, and the electric power consumption and the installation space are required to be decreased.
In order to overcome the restrictions as described above, although a membrane distillation method (Patent Literature 1) has been proposed, the membrane distillation method has the following problems. That is, the problems are: since some type of effluent to be treated is volatile, and the effluent as described above cannot be removed by distillation or membrane distillation; when wastewater containing hardness components is evaporated, a scale trouble occurs; since effluent generally contains organic substances, such as proteins, fouling occurs, and as a result, the membrane distillation performance is degraded; and since a basic operation is distillation, the energy consumption amount is large.
Although a method for performing a membrane-type activated sludge treatment (Patent Literature 2) has been proposed as a pre-treatment of the membrane distillation, in this method, there have been problems in that, for example, when operation conditions are out of the range of appropriate values, microorganisms are liable to be deactivated, and when once deactivated, the microorganisms are not returned to the original conditions; and since ⅓ to ½ of organic substances are changed into sludge by an activated sludge, a sludge containing precious water is discharged as a waste.
As an apparatus which resolves the problems described above, a water recovery apparatus (Patent Literature 3) formed of a hardness-component rough removing device, a softening device, an electrolysis device, a catalyst decomposition device, and an electrodialysis device has been proposed.
However, even by this water recovery apparatus, there have been still the following problems. That is, for example, since the current efficiency of the electrolysis device low, and the electric power consumption is large, more improvement thereof is required; since a mixed gas of oxygen/hydrogen is generated in the electrolysis device, and in addition, since chlorine oxides, such as hypochlorous acid, chloric acid, and perchloric acid, each of which is the load on the electrodialysis device provided at a latter stage, are generated, a countermeasure means therefor is required to be provided; in order to treat organic substances which cannot be removed by electrolysis using the electrolysis device and oxide substances, such as perchloric cid, generated thereby, the catalyst decomposition device is required to be provided at a latter stage of the electrolysis device; in consideration of the installation space, the maintenance, and the like, a simpler structure is desired; and in the electrodialysis device, since an acid and an alkali are directly manufactured, the water recovery rate of the entire system is decreased to a low level.
On the other hand, although a treatment of water containing organic substances and reducing substances by electrolysis under high temperature and high pressure conditions (Patent Literature 4) has been known, the application of this treatment to the water recovery in a closed space and the decomposition of urea have not been suggested, and furthermore, problems, such as influences on the treatments at a former stage and/or a latter stage of the water recovery performed in a closed system space, which may arise when the system is formed have not been disclosed at all.
In order to provide a water recovery apparatus in which wastewater containing scale components, organic substances, inorganic ions, and the like, in particular wastewater, such as human effluent and living wastewater, generated in a closed system space, such as a nuclear shelter, a hazardous shelter, a space station or a moon-Mars mission manned spacecraft, or a lunar base, is efficiently treated by a simple structural device without having concerns about clogging caused by the scale generation and fouling caused by the organic substances and without consuming a large amount of energy such as that of evaporation, intensive research was carried out by the present inventors, and it was found that after those types of wastewater described above are treated by a softening device to sufficiently remove hardness components, when oxidizable substances, such as an organic substance and ammonia, are electrolyzed by electrolysis under high temperature and high pressure conditions, and subsequently, ions are removed by an electrodialysis device, product water and a salt concentrated liquid can be obtained, that is, it was found that in the case in which in order to decompose oxidizable substances, such as an organic substance, urea, and ammonia, in the wastewater, electrolysis is performed under high temperature and high pressure conditions, the above problems can be resolved by the following operation mechanism, and hence, the patent application was submitted in the past (Japanese Unexamined Patent Application Publication No. 2015-80778, hereinafter, referred to as “prior application”).
By the electrolysis performed under high temperature and high pressure conditions, oxidizable substances in the wastewater can be changed into ions of carbonic acid, organic acids, nitric acid, and the like, each of which can be directly removed by the electrodialysis device provide at a latter stage.
By this electrolysis performed under high temperature and high pressure conditions, organic substances in the wastewater are partially electrolyzed into a carbon dioxide gas, and ammonia and nitric acid are partially electrolyzed into a nitrogen gas. Hence, the catalyst decomposition device provided at a latter stage of the electrolysis device in Patent Literature 3 can be omitted. In addition, under a high pressure condition, a gas generated by the electrolysis is dissolved in water by the pressure, and contact inhibition of the oxidizable substances to an electrode surface caused by air bubbles can be suppressed. In addition, since the treatment is performed at a high temperature, a pyrolytic effect can be used, and in addition, a material transport rate can be increased, so that the electrolysis efficiency can also be increased. Furthermore, since a reaction in which a hydrogen gas and an oxygen gas generated by the electrolysis of water are again returned to water can be performed, from a highly explosive mixed gas of hydrogen/oxygen, the oxygen concentration can be decreased, and by-product gases each can be made safe to have a value lower than the explosion limit thereof, and furthermore, the water recovery rate can also be increased. In addition, since the generation of oxides by the electrolysis is suppressed, the load on the electrodialysis device provided at a latter stage of the electrolysis device can be reduced.
As the electrodialysis device of the water recovery apparatus according to the prior application, a desalting electrodialysis device and an acid/alkali manufacturing electrodialysis device are preferably provided in series.
In this case, as shown in FIG. 4, the desalting electrodialysis device is a two-chamber type electrodialysis device in which between an anode and a cathode, repeating units including a concentration chamber, an anion exchange membrane AM, a desalting chamber, a cationic exchanged membrane CM, a concentration chamber, - - - are provided with an electrode chamber and a bipolar membrane BPM at each side so that the concentration chambers are provided at the respective electrode sides. In the desalting electrodialysis device, an anion X− and a cation Y+ forming a salt (XY) in water to be treated which is allowed to pass through the desalting chambers are concentrated in the respective concentration chambers through the anion exchange membrane AM and the cation exchange membrane CM, respectively, so that desalted water is obtained from the desalting chamber, and in addition, from the concentration chamber, a salt concentrated liquid is obtained.
On the other hand, in general, the acid/alkali manufacturing electrodialysis device is a three-chamber type electrodialysis device, and as shown in FIG. 5, between an anode and a cathode, repeating units including an acid chamber, an anion exchange membrane AM, a desalting chamber, a cation exchange membrane CM, an alkali chamber, - - - are provided with an electrode chamber and a bipolar membrane BPM at each side so that the acid chamber is provided at an anode side, and the alkali chamber is provided at a cathode side. As shown in FIG. 5, an anion X− and a cation Y+ in water to be treated are moved into the acid chamber and the alkali chamber through the anion exchange membrane AM and the cation exchange membrane CM, respectively, so that desalted water is obtained from the desalting chamber, and in addition, an acid solution and an alkali solution are also obtained from the acid chamber and the alkali chamber, respectively. That is, since the chambers adjacent to the desalting chamber are not the concentration chambers in which the anion X− and the cation Y+ are concentrated but are the acid chamber in which anions are only concentrated to generate H+ from the water and the alkali chamber in which cations are only concentrated to generate OH− from the water, the structure of the acid/alkali manufacturing electrodialysis device is different from that of the desalting electrodialysis device.
Patent Literature 1: Japanese Patent Publication 2006-095526A
Patent Literature 2: Japanese Patent Publication 2010-119963A
Patent Literature 3: Japanese Patent Publication 2013-075259A
Patent Literature 4: Japanese Patent 3746300 B
Patent Literature 5: Japanese Patent Publication 2015-80778A
According to the water recovery apparatus of the prior application, wastewater containing scale components, organic substances, inorganic ions, and the like can be efficiently treated by a simple structural device without having concerns about clogging caused by the scale generation and fouling caused by the organic substances and without consuming a large amount of energy such as that of evaporation; however, there have been the following troubles.
<Problem Caused by High-Temperature and High-Pressure Electrolysis Device>
i) By electrolysis, gases, such as a hydrogen gas derived from the electrolysis of water and carbon dioxide derived from the electrolysis of organic substances, are generated. In addition, a small amount of dissolved gas, such as oxygen, is present in original water. Since the explosion risk is generated by mixing of a hydrogen gas and oxygen, a safety countermeasure is required. By a high-temperature and high-pressure electrolysis device, although the amount of an oxygen gas generated thereby can be reduced, the generation of a hydrogen gas and a carbon dioxide gas cannot be completely stopped.
When a gas generated in the high-temperature and high-pressure electrolysis device is mixed as air bubbles in the electrodialysis device provided at a latter stage, the bubbles thus formed function as the resistance of the electrodialysis, and as a result, the voltage is increased. Furthermore, the amount of air bubbles thus generated is increased, and in a closed system, in order to secure the volume corresponding to the volume of the air bubbles thus generated, the size of the device is increased.
ii) The electric power consumption is large.
<Problems Caused by Electrodialysis Device>
1) By the electrodialysis device provided at one stage, treated water having a sufficient water quality cannot be obtained.
2) From the above (1), in the case in which an electric deionizing device is provided at a latter stage of the electrodialysis device, treated water having a high water quality can be obtained; however, the electric power consumption is excessively increased.3) According to the prior application, although the desalting electrodialysis device and the acid/alkali manufacturing electrodialysis device are provided, in this case, between the desalting electrodialysis device and the acid/alkali electrodialysis device, a tank is required to be provided.
In order to increase the water quality of the treated water, the desalting electrodialysis devices are necessarily provided at two stages, and in this case, three electrodialysis devices, that is, two desalting electrodialysis devices and one acid/alkali electrodialysis device, are required.
4) As is the electrolysis device, also in the electrodialysis device, a hydrogen gas is generated in a cathode chamber, and an oxygen gas is primarily generated in an anode chamber by the electrolysis of water, so that a problem similar to that of the electrolysis device occurs.